Conventional piston rings are usually in one-piece structure with an opening and leakage inevitably occurs at the opening. And, the higher the working pressure is, the more leakage there will be. As the abrasion increases, the leakage gradually becomes worse. Thus, the conventional piston ring of one-piece structure with an opening has always been working under leakage, and due to the opening leakage, a lot of power is lost and the efficiency is decreased.
In order to solve the leakage problem of the conventional piston ring with an opening, multiple piston rings are usually adopted so as to produce a labyrinth seal effect and reduce leakage. The labyrinth seal can decrease leakage, but can not avoid leakage. Moreover, multiple ring grooves are needed for the multiple piston rings. The increase in the number of ring grooves will surely lead to the reduction in the structural strength of the piston.
Conventional piston usually has two to three piston rings and one oil ring. During the operation of the piston, the lubricating oil is sputtered to the wall of the cylinder by the movement of the crankshaft to lubricate the piston rings. However, with the increase in the number of piston rings, friction gradually increases. The increase of each piston ring further deteriorates the lubrication of the upper ring. The top ring withstands the largest stress, while the lubrication condition thereof is the worst, especially near the upper dead center, which is in the state of dry friction. Inadequate lubrication is the main cause for the leakage and abrasion at the working surface.
Directing at the leakage problem of the conventional piston ring of one-piece structure with an opening, many forms of two-piece assembled piston rings successively appears. For example, Chinese patents 200410040819.7, 94221414.5 and 200710078397.6 respectively disclose an assembled piston ring, and their technical solutions are to place two pieces of piston rings up and down in the original piston ring groove, with the openings of the two piston rings staggered (the openings not arranged in the same direction), so as to achieve the purpose of mutual sealing. The two-piece assembled piston ring is expected to achieve a complete seal by the assembly of the two pieces, but such expected effect actually is not achieved.
Typically, the gap between the upper plane/surface of the piston ring and the piston ring groove is called an upper side gap, and the gap between the lower plane/surface of the piston ring and the piston ring groove is called a lower side gap. The gap between the piston ring groove and the side opposite to the working surface (i.e. the surface in contact with the wall of the cylinder) of the piston ring is called a back gap. The conventional one-piece piston ring with an opening and the two-piece assembled piston ring leak mainly through three channels.
The first leakage channel: the opening of the conventional one-piece piston ring is directly connected to the high pressure area and the low pressure area, obviously forming a direct leakage channel. The leakage amount of the leakage channel depends on the size of the opening, and the amount of leakage is intensified with the increasing abrasion.
The second leakage channel: when the two-piece assembled piston ring works under pressure, the opening of the upper piston ring and the upper side gap of the upper piston ring is connected to the common back gap of the two-piece piston ring. The common back gap is further connected to the opening of the lower piston ring. The opening of the lower piston ring is directly connected to the low pressure area. Hence, an indirect leakage channel is formed; the leakage amount of this channel depends on the size of the opening and is intensified with the increasing abrasion.
The third leakage channel: leakage occurs at the working surface of the piston ring. The working surface remains sealed by the lubricating oil. For multiple rings, the top ring works under the worst lubrication condition, and thus is worst sealed. When the working pressure exceeds the sealability of the working surface, leakage begins. The leakage amount of this channel depends on the thickness and the lubrication of the working surface, and is intensified with the increasing pressure.
Thus, it can be seen from above that, a complete seal cannot merely rely on the assembly of two pieces of piston rings. The two-piece assembled piston ring only solves the leakage problem of the first leakage channel, but cannot solve the leakage problem of the second and the third leakage channels.
In a two-piece assembled piston ring, the two pieces of piston rings are installed in the original piston ring groove. Hence, each piece of piston ring is only ½ of the thickness of the original piston ring, which will surely reduce the structural strength. A piston ring works at high temperature, high pressure and high speed, so the reduction in structural strength will directly affect its reliability. If the total thickness of the assembly of two pieces of piston rings increases, the lubrication of the upper piston ring will be further deteriorated, and the sealing effect cannot be ensured due to insufficient lubrication. Thus, sufficient lubrication is important to ensure the sealing effect.
Furthermore, the piston rings produce the phenomenon of oil pumping in the piston ring groove. When the piston ring reciprocates with the piston, the upper side gap and the lower side gap are alternately closed. Thus, the lubricating oil pumps into the back gap via the lower side gap, and further pumps into the upper side gap via the back gap. After completing the tasks of lubricating and cleaning, the lubricating oil runs into the combustion chamber and forms carbon deposits because the lubricating oil is not completely burned. In this process, the lubricating and cleaning function of the lubricating oil is beneficial, but the formation of carbon deposits is harmful.
In addition, a conventional piston has to provide multiple piston ring grooves in the limited size, so as to corporate with the multi-piston rings and further achieve the sealing effect. It not only makes the processing procedures of the conventional piston complicated, but also reduces the strength of the piston. Plus, a plurality of piston rings are needed to achieve the sealing effect, which leads to the conventional piston and piston rings complicated in processing, low in reliability, and increased in costs.